Piping and vessels are subjected to environmental cracking over their useful service life. Environmental cracking is caused by a reaction between the alloy or steel used to make up the piping or vessel and the local environment, with the pipe or vessel under stress. Environmental cracking is different than fatigue cracking, which is caused solely by cyclical mechanical loading. Environmental cracking can occur at much lower levels of stress than fatigue cracking. Some types of environmental cracking include stress corrosion cracking, sulfide stress cracking and hydrogen induced cracking.
Environmental cracking is a concern because it can cause catastrophic equipment failure, resulting in damage to the environment, loss of life and loss of production from the equipment. As an example, stress corrosion cracking has been responsible for numerous pipeline failures. Thus, there is a need to develop non-destructive techniques that will detect areas of a pipe or vessel that have been subjected to environmental cracking. Such detection would permit corrective measures to be taken to prevent catastrophic equipment failure.
Environmental cracking is difficult to detect with conventional non-destructive techniques. Some techniques, such as magnetic particle testing and dye penetrant testing, are capable of detecting environmental cracking, but only with direct access to the wall of the piping or vessel. However, many testing situations exist where direct access to the wall is not possible. For example, direct access is not possible when searching for stress corrosion cracking on the exterior surface of a buried pipeline. Nor is direct access possible when searching for sulfide stress cracking on the interior surface of a vessel that is still in service.
Some conventional non-destructive testing techniques do not require direct access to the cracked surface. However, these techniques are not effective in detecting environmental cracking. For example, stress corrosion cracks are typically compact and filled with corrosion products. It is believed that this makes detection difficult with conventional shear wave (or angle beam) ultrasonic testing or with radiographic testing. In addition, stress corrosion cracking in pipelines is oriented longitudinally. This makes the stress corrosion cracking undetectable with magnetic flux leakage testing that is implemented with in-line inspection pigs, because the magnetic field lines run parallel to the cracks.